Crystal unit

ABSTRACT

A crystal unit includes an AT-cut crystal element, excitation electrodes, extraction electrodes. The AT-cut crystal element has an approximately rectangular planar shape. The excitation electrodes are disposed on front and back of principal surfaces of the AT-cut crystal element. The extraction electrodes are extended from the excitation electrodes to a side of one side of the AT-cut crystal element via a side surface of the AT-cut crystal element. Assuming that an extraction angle of the extraction electrode from the principal surface to the side surface is defined as an angle θ with respect to an X-axis of a crystallographic axis of a crystal, the angle θ is equal to or greater than 59 degrees and equal to or less than 87 degrees.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 U.S.C. §119 toJapanese Patent Application No. 2015-173455, filed on Sep. 3, 2015, theentire content of which is incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to a crystal unit using an AT-cut crystalelement.

DESCRIPTION OF THE RELATED ART

As downsizing of an AT-cut crystal unit proceeds, it has becomedifficult to manufacture crystal elements for crystal units by amanufacturing method of mechanical processing. Accordingly, an AT-cutcrystal element manufactured using photolithography technique and wetetching technique has been developed.

Such AT-cut crystal element is, for example, secured to and mounted in acontainer such as a ceramic package with a conductive adhesive or asimilar adhesive, thus ensuring configuring a crystal unit. To meetspecifications for CI (crystal impedance), this type of crystal unit hasbeen variously devised.

For example, as disclosed in, for example, FIG. 10A to FIG. 11B inJapanese Unexamined Patent Application Publication No. 2014-27505,inclined portions are formed at end portions of a crystal element atwhich the crystal element decreases in thickness. The crystal element issecured to a container at these inclined portions with conductiveadhesive. With this structure, vibration energy at an excitation portionof the crystal element can be cut off between the excitation portion andthe inclined portions, thereby reducing a deterioration of a property ofa crystal unit can be expected.

For example, Japanese Unexamined Patent Application Publication No.2014-11650 discloses that extraction electrodes extracted fromexcitation electrodes, which are disposed on both principal surfaces ofa crystal element, to mounting portions are extended to an m surface,which is a side surface of the crystal element, of a crystal. With thisstructure, since the extraction electrodes are not present directly in apropagation direction of vibrations, this ensures a reduction in aleakage of energy at the excitation portions via the extractionelectrodes, achieving an improvement in CI.

However, the Patent Literatures do not describe a preferable conditionon a wiring direction of an extraction electrode and a preferable sidesurface shape of the crystal element in the above-described conventionalstructures. According to study on these applications by the inventor ofthe disclosure, it has been proved that focusing on the condition andthe shape can improve properties of a crystal unit.

A need thus exists for a crystal unit which is not susceptible to thedrawback mentioned above.

SUMMARY

There is provided a crystal unit that includes an AT-cut crystalelement, excitation electrodes, and extraction electrodes. The AT-cutcrystal element has a planar shape which is approximately a rectangularshape. The excitation electrodes are disposed on front and back ofprincipal surfaces of the AT-cut crystal element. The extractionelectrodes are extended from the excitation electrodes to a side of oneside of the crystal element via a side surface of the AT-cut crystalelement. Assuming that an extraction angle of the extraction electrodefrom the principal surface to the side surface is defined as an angle θwith respect to an X-axis of a crystallographic axis of a crystal of theAT-cut crystal element, the angle θ is equal to or greater than 59degrees and equal to or less than 87 degrees.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and additional features and characteristics of thisdisclosure will become more apparent from the following detaileddescription considered with reference to the accompanying drawings,wherein:

FIG. 1A to FIG. 1E are explanatory drawings of an AT-cut crystal element10 provided with a crystal unit according to an embodiment;

FIG. 2A and FIG. 2B are explanatory drawings of especially a thirdinclined portion and a fourth inclined portion of the crystal element10;

FIG. 3A to FIG. 3E are explanatory drawings of an excitation electrodeand an extraction electrode of the crystal unit according to theembodiment;

FIG. 4A to FIG. 4C are drawings describing a structure of the crystalunit;

FIG. 5A, FIG. 5B, and FIG. 5C are explanatory drawings of an example ofa method for manufacturing the crystal element 10;

FIG. 6A, FIG. 6B, and FIG. 6C are explanatory drawings of the example ofthe method for manufacturing the crystal element 10 continuous from FIG.5C;

FIG. 7A, FIG. 7B, and FIG. 7C are explanatory drawings of the example ofthe method for manufacturing the crystal element 10 continuous from FIG.6C;

FIG. 8A and FIG. 8B are explanatory drawings of the example of themethod for manufacturing the crystal element 10 continuous from FIG. 7C;

FIG. 9 is an explanatory drawing of the example of the method formanufacturing the crystal element 10 continuous from FIG. 8B;

FIG. 10A to FIG. 10C are drawings especially describing an etching stateof the example of the method for manufacturing the crystal element 10;

FIG. 11A and FIG. 11B are explanatory drawings of first to thirdsurfaces of the crystal element 10; and

FIG. 12A and FIG. 12B are explanatory drawings describing an effect ofan extraction angle of an extraction electrode of the crystal unitaccording to the embodiment.

DETAILED DESCRIPTION

The following description describes embodiments of a crystal unitaccording to this disclosure with reference to the drawings. Eachdrawing used in the description is merely illustrated schematically forunderstanding this disclosure. In each drawing used in the description,like reference numerals designate corresponding or identical elements,and therefore such elements may not be further elaborated here. Shapes,dimensions, materials, and a similar factor described in the followingexplanations are merely preferable examples within the scope of thisdisclosure. Therefore, this disclosure is not limited to only thefollowing embodiments.

1. Structure of AT-Cut Crystal Element

First, the following description describes an AT-cut crystal element 10provided with a crystal unit of this disclosure mainly with reference toFIG. 1A to FIG. 2B. FIG. 1 A to FIG. 1E are explanatory drawings of thiscrystal element 10. Especially, FIG. 1A illustrates a plan view of thecrystal element 10, and FIG. 1B to FIG. 1E are sectional drawings of thecrystal element 10 each taken along the line P-P, the line Q-Q, the lineID-ID, and the line IE-IE in FIG. 1A. FIG. 2A and FIG. 2B illustrateparts illustrated in FIG. 1D in further detail. Especially, FIG. 2Billustrates enlarged N part (namely, a third inclined portion) in FIG.2A.

Here, each of coordinate axes X, Y′, and Z′ shown in FIG. 1A and FIG. 1Dare crystallographic axes of a crystal in the AT-cut crystal element 10.The AT-cut crystal element itself is described in, for example,literature: “Handbook of Quartz Crystal Device” (Fourth Edition, page 7or other pages, published by Quartz Crystal Industry Association ofJapan, March 2002) in detail. Therefore, the explanation will beomitted.

The crystal element 10 of this embodiment is an AT-cut crystal elementhaving a planar shape which is approximately a rectangular shape. Thecrystal element 10 is secured to a container (see 30 in FIG. 4A and FIG.4B) with securing members (see 32 in FIG. 4A and FIG. 4B) at a firstside 10 a side, which is one side of the crystal element 10. Thiscrystal element 10 includes a first inclined portion 12, second inclinedportions 14, a first secured portion 22 a, and a second secured portion22 b. The first inclined portion 12 is inclined such that the crystalelement 10 decreases in thickness from the proximity of the first side10 a to this first side 10 a. The second inclined portions 14 aredisposed on respective both ends of the first side 10 a. The secondinclined portions 14 are formed integrally with the first inclinedportion 12. The second inclined portions 14 are inclined gentler thanthe first inclined portion 12. The first secured portion 22 a and thesecond secured portion 22 b are formed integrally with the secondinclined portions 14. The first secured portion 22 a and the secondsecured portion 22 b each project out from the first side 10 a tooutside the crystal element 10 to be used for securing with the securingmembers.

The crystal element 10 of this embodiment is an approximatelyrectangular-shaped crystal element whose first side 10 a is parallel toa Z′-axis of a crystal and a second side 10 b and a third side 10 c,which intersect with the first side 10 a, are parallel to an X-axis ofthe crystal and are long in the X-axis direction.

Accordingly, the first and the second secured portions 22 a and 22 b ofthis embodiment each project out in a direction parallel to the X-axisof the crystal. Moreover, the first and the second secured portions 22 aand 22 b of this embodiment each have a convex shape having twoprotrusions 22 x, which convexly project out in a direction parallel tothe X-axis of the crystal.

Compared with the first inclined portion 12, the second inclinedportions 14 are inclined gently. Therefore, a thickness t2 (FIG. 1C) ofthe second inclined portion 14 in a direction parallel to a Y′-axis ofthe crystal is thicker than a thickness t1 (FIG. 1B) of the firstinclined portion 12 in the identical direction. These second inclinedportions 14 are parts continuous to the first and the second securedportions 22 a and 22 b; therefore, the thick thickness of the secondinclined portions 14 contributes to an improvement in an impactresistance of the crystal unit after the crystal element 10 is securedto the container.

In this embodiment, the crystal element 10 is secured to the containerat an end portion on a +X-side of the crystal element 10; however, thecrystal element 10 may be secured at a −X-side of the crystal element10. Note that, a dimension of the first inclined portion 12 in the Xdirection is longer than that of a fifth inclined portion 20.Accordingly, disposing the first and the second secured portions at theend portions on the +X-side of the crystal element 10 easily widensbetween excitation electrodes 26 and the secured portions 22 a and 22 b,thereby preferable in terms of improvement in CI. This disclosurefeatures an extraction angle θ of an extraction electrode 28 and a sidesurface shape of the crystal element 10, which will be described later.Therefore, the secured portions 22 a and 22 b are not necessary, and thecrystal element 10 may be secured to the container at the first inclinedportion 12 without disposing the secured portions 22 a and 22 b.

The first inclined portion 12 and the second inclined portions 14 ofthis embodiment each have a structure inclined in two stages along theX-axis direction of the crystal (see FIG. 1B and FIG. 1C). Note that,the numbers of stages of the inclined portions are not limited to this.It is only necessary that an inclined portion be inclinedly connected toa principal surface 10 d of the crystal element 10. The principalsurface of the crystal element 10 is a region excluding the first to thefifth inclined portions 12 to 20 of the crystal element 10 and is aregion corresponding to an X-Z′ plane of the crystal.

The crystal element 10 of this embodiment includes a third inclinedportion 16 and a fourth inclined portion 18. The third inclined portion16 and the fourth inclined portion 18 are inclined such that the crystalelement 10 decreases in thickness from the proximities of the respectivesecond side 10 b and third side 10 c, which are two sides intersectingwith the first side 10 a, to these sides 10 b and 10 c.

These third inclined portion 16 and fourth inclined portion 18 each havethree surfaces, first to third surfaces 24 a, 24 b, and 24 c in thisembodiment (FIG. 1D). The first surface 24 a is a surface intersectingwith the principal surface 10 d of this crystal element 10. Moreover,the first surface 24 a is a surface corresponding to a surface where theprincipal surface 10 d is rotated by θ1 (see FIG. 2B) with the X-axis ofthe crystal as a rotation axis. Further, in this embodiment, the firstsurface 24 a, the second surface 24 b, and the third surface 24 cintersect in this order. Moreover, the second surface 24 b is a surfacecorresponding to a surface where the principal surface 10 d is rotatedby θ2 (see FIG. 2B) with the X-axis of the crystal as a rotation axis.The third surface 24 c is a surface corresponding to a surface where theprincipal surface 10 d is rotated by θ3 (see FIG. 2B) with the X-axis ofthe crystal as a rotation axis.

Although details of these angles θ1, θ2, and θ3 will be described laterin the “4. Explanation of Experimental Results” section, the followingdescription has been found to be preferable: θ1=4°±3.5°, θ2=−57°±5°,θ3=−42°±5° and more preferably θ1=4°±3°, θ2=−57°±3°, and θ3=−42°±3°.

In the crystal element 10 of this embodiment, respective two sidesurfaces (Z′ surfaces) intersecting with the Z′-axis of the crystal(namely, the third inclined portion 16 and the fourth inclined portion18) have a relationship of point symmetry around a center point 0 (seeFIG. 2A) of the crystal element 10. The point symmetry mentioned hereincludes a point symmetry regarded as substantially identical even ifthe shapes slightly differ. Compared with the case of not having therelationship of point symmetry, with this point symmetry, the crystalunit exhibits good property, and therefore the point symmetry ispreferable.

The crystal element 10 of this embodiment includes the fifth inclinedportion 20 on a side at a side opposed to the first side 10 a. Thisfifth inclined portion 20 is an inclined portion where the crystalelement decreases in thickness as the crystal element approaches thisside (see FIG. 1B and FIG. 1C).

2. Configurations of Electrodes and Crystal Unit

The following description describes configurations of the excitationelectrode 26 and the extraction electrode 28 mainly with reference toFIG. 3A to FIG. 4C and an overall configuration of the crystal unit.FIG. 3A to FIG. 3E illustrate the crystal element 10, which isillustrated in FIG. 1A, that includes the excitation electrodes 26 andthe extraction electrodes 28. Especially, FIG. 3A is a plan view of thecrystal element 10 including these electrodes. FIG. 3B to 3E aresectional drawings of the crystal element 10 each taken along the lineP-P, the line Q-Q, the line R-R, and the line S-S in FIG. 3A. FIG. 4A to4C illustrate the crystal element 10 with the electrodes 26 and 28mounted to the container 30. Especially, FIG. 4A is a plan view of thecrystal element 10. FIG. 4B and FIG. 4C are sectional drawings eachtaken along the line P-P and the line Q-Q in FIG. 4A.

In this embodiment, the excitation electrodes 26 are disposed onrespective front and back of the principal surfaces 10 d of the crystalelement 10. The extraction electrode 28 is disposed from the excitationelectrode 26 to the corresponding secured portion of the first securedportion 22 a or the second secured portion 22 b via the correspondinginclined portion of the third inclined portion 16 or the fourth inclinedportion 18. Moreover, the extraction electrode 28 is extracted via thefirst surface 24 a, which is the inclined portion, corresponding to thethird inclined portion 16 or the fourth inclined portion 18.Specifically, the excitation electrode 26 on the front surface side ofthe principal surface 10 d in FIG. 3A reaches the first secured portion22 a via the first surface 24 a of the third inclined portion 16. Theexcitation electrode 26 on the back surface side of the principalsurface 10 d in FIG. 3A reaches the second secured portion 22 b via thefirst surface 24 a of the fourth inclined portion 18. Accordingly, thisextraction structure can prevent the extraction electrodes 28 fromreaching the secured portions 22 a and 22 b directly via over the firstinclined portion 12 and the second inclined portions 14.

The following description has been found through experiments by theinventor. In the case where an extraction angle of the extractionelectrode 28 from the principal surface 10 d to the third or the fourthinclined portion 16 or 18 is defined as an angle θ with respect to theX-axis of the crystallographic axis of the crystal (see FIG. 3A), thisextraction angle θ is preferably: equal to or greater than 59 degreesand equal to or less than 87 degrees. More preferably, this θ is: equalto or greater than 62 degrees and equal to or less than 75 degrees.Further preferably, this θ is: equal to or greater than 64 degrees andequal to or less than 74 degrees. This configuration ensures improvingCI (crystal impedance) of the crystal unit. The details will bedescribed later in the “4. Explanation of Experimental Results” section.

As illustrated in FIG. 4A, FIG. 4B and FIG. 4C, the crystal element 10with the excitation electrodes 26 and the extraction electrodes 28 ismounted inside a concave portion 30 a of, for example, the ceramicpackage 30 as the container. A frequency adjustment or a similaroperation is performed on the crystal element 10 and a lid member (notillustrated) seals the crystal element 10, thus ensuring configuring thecrystal unit. Specifically, the secured portions 22 a and 22 b and apart of the first inclined portion 12 and the second inclined portions14 of the crystal element 10 and securing pads 30 b of the container 30are secured with the securing members (for example, conductiveadhesives) 32. Then, the crystal unit can be configured through thefrequency adjustment and the sealing. As shown in FIG. 4B, the crystalunit has a mounting terminal 30 c.

3. Example of Method for Manufacturing AT-Cut Crystal Element 10

Next, a description will be given of the example of the method formanufacturing the AT-cut crystal element 10 provided with the crystalunit according to the embodiment with reference to FIG. 5A to FIG. 8B. Alarge number of the crystal elements 10 can be manufactured from aquartz-crystal wafer by photolithography technique and wet etchingtechnique. Accordingly, some drawings in drawings used to explain theexample of the manufacturing method include plan views of aquartz-crystal wafer 10 w and enlarged plan views of a part M of thequartz-crystal wafer 10 w. Further, some drawings in the drawings usedto explain the example of the manufacturing method also includesectional drawings. In all drawings using the sectional drawings in FIG.5A to FIG. 8B, the sectional drawings taken along the line P-P in FIGS.5A, 6A, 7A, and 8A are illustrated in FIGS. 5B, 6B, 7B, and 8B, and thesectional drawings taken along the line Q-Q in FIGS. 5A, 6A, and 7A areillustrated in FIGS. 5C, 6C, and 7C.

In the example of the manufacturing method, first, the quartz-crystalwafer 10 w is prepared (FIG. 5A). While, as it is well known, theoscillation frequency of the AT-cut crystal element 10 is approximatelydetermined by the thickness of the principal surface (the X-Z′ surface)part of the crystal element 10, the quartz-crystal wafer 10 w is a waferwith a thickness T (see FIG. 5B) thicker than the final thickness t (seeFIG. 7B) of the crystal element 10.

Next, the well-known photolithography technique is used to form etchingresist masks 40, which are masks to foam the outer shape of the crystalelement, on both front and back surfaces of the quartz-crystal wafer 10w. The etching resist masks 40 according to the embodiment areconfigured of a part corresponding to the outer shape of the crystalelement, a frame part that holds each crystal element, and a connectingportion that connects the crystal element and the frame part (a partindicated by 10 x in FIG. 5A). The etching resist masks 40 are formed tobe opposed to one another on the front and back of the quartz-crystalwafer 10 w.

The quartz-crystal wafer 10 w after the etching resist masks 40 areformed is dipped in an etching solution mainly composed of hydrofluoricacid for a predetermined period. This process dissolves parts of thequartz-crystal wafer 10 w without being covered with the etching resistmasks 40 to provide the rough outer shape of the crystal element 10.

Next, the etching resist masks 40 are removed from the quartz-crystalwafer 10 w. Then, the example of the manufacturing method removes onlythe parts of the etching resist masks 40 corresponding to the crystalelement 10 and connecting portions 10 x and leaves the partcorresponding to the frame portion (FIG. 6A).

Next, this quartz-crystal wafer 10 w is dipped again in the etchingsolution mainly composed of hydrofluoric acid for the predeterminedperiod. Here, the predetermined period is a period during which thethickness t (FIG. 7B) of a forming scheduled region for the crystalelement 10 can satisfy the specification of an oscillation frequencyrequired to the crystal element 10, and the Z′-side surface of thecrystal element 10 can be constituted of the first to the third surfaces24 a to 24 c according to the disclosure. The period can be determinedby experiments in advance. The experiments performed by the inventorhave found that, as the etching proceeds, the Z′ surface of the crystalelement 10 changes its shape. FIG. 10A to FIG. 10C are explanatorydrawings illustrating a part of the quartz-crystal wafer 10 w and aresectional drawings illustrating the change in shape according to anamount of etching to a part corresponding to the third inclined portionof the crystal element. The following description has been found. As theetching proceeds, the state changes: a protrusion 1 Oz remains asillustrated in FIG. 10A, the quartz-crystal wafer 10 w is constituted offour surfaces of first surface 10 g, second surface 10 h, third surface10 i and fourth surface 10 j (a fourth surface generating state) asillustrated in FIG. 10B, and the quartz-crystal wafer 10 w isconstituted of three surfaces of the first to the third surfaces 24 a,24 b, and 24 c according to this disclosure (the state of thisdisclosure) as illustrated in FIG. 10C in this order. Moreover, theexperiments have found that, to obtain the side surfaces constituted ofthe three surfaces of the first to the third surfaces of thisdisclosure, in the case where the etching is performed in thepredetermined etchant, the etching temperature, and a similar condition,it is necessary to perform the etching on the quartz-crystal wafer 10 wuntil the quartz-crystal wafer 10 w has the thickness in a range of 55%to 25% with respect to an initial thickness T. Therefore, the initialthickness T, the above-described etching period, and a similar factorare determined such that the specification of the oscillation frequencyand the three surfaces of the first to the third surfaces are obtained.

Next, the etching resist masks are removed from the quartz-crystal waferafter the above-described etching is performed to expose a crystalsurface (not illustrated). Then, a well-known film formation method isused to form a metal film (not illustrated) for forming excitationelectrodes and extraction electrodes of the crystal unit on the entiresurface of the quartz-crystal wafer. Next, the well-knownphotolithography technique and metal etching technique are used toperform a patterning on the metal film in an electrode shape to form theexcitation electrodes 26 and the extraction electrodes 28 (FIG. 8A andFIG. 8B). This ensures obtaining a crystal unit that includes thecrystal element 10, the excitation electrodes 26, and the extractionelectrodes 28.

In a state illustrated in FIG. 8B, the crystal element 10 is connectedto the quartz-crystal wafer 10 w via the connecting portions 10 x.Therefore, at first, an appropriate external force F (FIG. 9) is appliedto the connecting portions 10 x to separate the crystal elements 10 fromthe quartz-crystal wafer 10 w at, for example, the centers of theconnecting portions 10 x and individually dice the crystal elements 10(FIG. 9). With this disclosure, since the connecting portion 10 x has anopening at the center, after this individual dicing process, the partsof the connecting portions 10 x remaining on the crystal element 10 sideare actively used as the first and the second secured portions 22 a and22 b. Devising a design of the connecting portion 10 x ensures obtainingthe second inclined portion.

Mounting the crystal element thus formed to the container 30 asillustrated in FIG. 4A to FIG. 4C ensures obtaining the crystal unitaccording to the embodiment.

4. Explanation of Experimental Results

4-1. First to Third Surfaces

The following description describes the first to the third surfaces 24a, 24 b, and 24 c with reference to FIG. 11A and FIG. 11B.

FIG. 11A is a drawing describing how the CI (the crystal impedance) ofthe crystal units configured of the crystal elements are differentdepending on the difference in the shape of the Z′ surfaces of thecrystal elements, namely, the difference in the shapes of the thirdinclined portion and the fourth inclined portion. FIG. 11A indicatessample numbers of the crystal elements used in the experiment andfeatures of the shape of the Z′ surface of each sample (featurescorresponding to FIG. 10A to FIG. 10C) on the horizontal axis, andindicates the CI (the relative value) on the vertical axis. Theoscillation frequency of the experimental sample is close to 38 MHz.

As apparent from FIG. 11A, among the sample where the protrusion remainson the Z′ surface of the crystal element, the sample where the Z′surface of the crystal element is constituted of the four first tofourth surfaces, and the sample according to this disclosure where theZ′ surface of the crystal element is constituted of the three, first tothird surfaces, the impedance of the sample according to this disclosureis found to be small. Accordingly, it has been found that the thirdinclined portion 16 and the fourth inclined portion 18, which areillustrated in FIG. 1A, are preferably the inclined portions constitutedof the first to the third surfaces 24 a to 24 c.

FIG. 11B is an explanatory drawing illustrating the first to the thirdsurfaces 24 a, 24 b, and 24 c according to this disclosure.Specifically, FIG. 11B indicates the experimental result by theinventors according to this application, and indicates the difference ofthe etching speed in various crystal surfaces of the crystal with ahydrofluoric acid-based etchant. More specifically, FIG. 11B indicatesthe angles where the AT-cut principal surface as a reference is rotatedwith the X-axis of the crystal as a rotation axis on the horizontalaxis, and indicates the etching speeds of the respective crystalsurfaces obtained by rotating an AT-cut plate as described above on thevertical axis. The etching speeds of the respective surfaces areindicated by the relative value as a reference etching speed of theAT-cut surface.

As apparent from FIG. 11B, it has found that the crystal has the maximumetching speed on each surface of a surface corresponding to a surfacewhere the AT-cut principal surface is rotated by θ1, a surfacecorresponding to a surface where the AT-cut principal surface is rotatedby θ2, and a surface corresponding to a surface where the AT-cutprincipal surface is rotated by θ3. Then, θ1 is near 4°, θ2 is near−57°, and θ3 is near −42°. Furthermore, the experiment by the inventorhas found that, in the region where the impedance is good as describedwith reference to FIG. 11A, the angles are: θ1=4°±3.5°, θ2=−57°±5°, andθ3=−42°±5°, and more preferably, θ1=4°±3°, θ2=−57°±3°and θ3=−42°±3°.Each surface specified by these θ1 to θ3 corresponds to the first to thethird surfaces according to this disclosure.

4-2. Extraction Electrode

The following description describes experimental results on a method forextracting the extraction electrode. The experiment focuses on thedimension of the excitation electrode 26 along the X direction of thecrystal axis and the length of the extraction electrode 28, which areillustrated in FIG. 3A to FIG. 3E. Sample groups with two levels, levelA: the X dimension of the excitation electrode 26 is long and the lengthof the extraction electrode 28 is short, and level B: compared withlevel A, the X dimension of the excitation electrode 26 is short and thelength of the extraction electrode 28 is long are used. The differencein CI (the crystal impedance) when the extraction angle θ of theextraction electrode was changed was examined.

FIG. 12A illustrates a relationship between the extraction angle θ andthe CI of the crystal unit in the sample group with level A. FIG. 12Billustrates a relationship between the extraction angle θ and the CI ofthe crystal unit in the sample group with level B. All drawings indicatethe extraction angle θ on the horizontal axis and the CI (the relativevalue) on the vertical axis.

The CI was examined on samples with four conditions, the extractionangle θ of 0 degrees, 45 degrees, 65 degrees, and 90 degrees, in bothlevel A and level B. The following description has been found. In bothlevel A and level B, compared with the case of the extraction angle of 0degrees, that is, the case where an extraction electrode goes throughneither the third inclined portion nor the fourth inclined portion, thecase where the extraction electrode goes through the third inclinedportion and the fourth inclined portion with the predeterminedextraction angle θ value in a range of 45 to 90 degrees produces a smallCI. Specifically, the following description has been found. In level A,the CI is smaller at the extraction angle θ of 69 degrees compared withother angles (FIG. 12A). In level B, the CI is smaller at the extractionangle θ of 74 degrees compared with other angles (FIG. 12B).

The range of the extraction angle in which CI worsens by 2% with respectto the CI values at these preferable angles of 69 degrees and 74 degreeswas examined. It has been found that the angle was equal to or greaterthan 59 degrees and equal to or less than 87 degrees in level A andequal to or greater than 62 degrees and equal to or less than 75 degreesin level B. Further examination of the range of the extraction angle inwhich the CI worsens by 1% with respect to the CI values at thesepreferable angles of 69 degrees and 74 degrees found that the angle wasequal to or greater than 64 degrees and equal to or less than 74 degreesin level A and equal to or greater than 63 degrees and equal to or lessthan 83 degrees in level B. These amounts of worsening such as 2% and 1%can be considered as a reference of a threshold of the CI value fordesigning crystal units; therefore, it is preferable to set theextraction angle θ within the respective ranges.

Accordingly, collectively considering the above-described examinationresults, to improve CI, it has been found that the extraction angle θ ofthe extraction electrode 28 is preferably equal to or greater than 59degrees and equal to or less than 87 degrees, more preferably equal toor greater than 62 degrees and equal to or less than 75 degrees, andfurther preferably equal to or greater than 64 degrees and equal to orless than 74 degrees.

5. Other Embodiments

The above-described example describes the structure where, asillustrated in FIG. 3A to FIG. 3E, the extraction electrode 28 goes fromthe excitation electrode 26 through only the third inclined portion 16or only the fourth inclined portion 18 with the extraction angle θ andthen reaches the second inclined portion 14 or the first inclinedportion 12. However, when the extraction electrode 28 goes through thethird inclined portion 16, a part of the extraction electrode 28 may beformed also on the first inclined portion 12 and may be extended. Forexample, “a part of” is around 10% or less of a width of the excitationelectrode 26 in the Z′ direction. Up to this extent, even if theextraction electrode 28 protrudes to the first inclined portion 12 sideand goes through the third inclined portion 16, vibration energy on theexcitation electrode 26 side is less likely to leak to the first securedportion 22 a side via the first inclined portion 12, thereby the CI doesnot substantially worsen.

The θ is more preferably equal to or greater than 62 degrees and equalto or less than 75 degrees. The θ is further preferably equal to orgreater than 64 degrees and equal to or less than 74 degrees. This isbecause of the following reason. The extraction angle θ at theabove-described predetermined value ensures reducing an amount of aleakage of vibration energy depending on the extraction angle of theextraction electrode compared with the case other than that.Accordingly, the improvement in CI is achieved. The details will bedescribed later in the “4. Explanation of Experimental Results” section.

To embody this disclosure, the following embodiment is preferable. Theside surface is an inclined portion inclined such that the crystalelement decreases in thickness to an end of the crystal element along aZ′-axis of the crystallographic axis of the crystal. The side surfacehas three, first to third surfaces. The first surface is an inclinedportion of a surface corresponding to a surface where an X-Z′ surfaceexpressed by the crystallographic axes of the crystal (the principalsurface) of the crystal element is rotated by 4±3.5° with an X-axis ofthe crystal as a rotation axis. The extraction electrode is extractedvia the first surface.

Such inclined portion ensures reducing an amount of a leakage ofvibration energy depending on the shape of the side surface intersectingwith the Z′-axis of the crystal element compared with the case otherthan that, thereby ensuring the improvement in CI.

In this preferable example, typically, the following embodiment ispreferable. The first surface, the second surface, and the third surfaceintersect in this order. The second surface is a surface correspondingto a surface where the principal surface is rotated by −57±5° with theX-axis of the crystal as a rotation axis. The third surface is a surfacecorresponding to a surface where the principal surface is rotated by−42±5° with the X-axis of the crystal as a rotation axis.

With the crystal unit according to the embodiments, the extraction angleof the extraction electrode is optimized, thereby ensuring theimprovement in CI compared with that of the conventional crystal unit.With the preferable example, the side surface shape of the crystalelement is optimized, thereby ensuring the improvement in CI also in theside surface shape compared with that of the conventional crystal unit.Combined with the extraction angle of the extraction electrode, thisfeature ensures improving CI compared with that of the conventionalcrystal unit.

The principles, preferred embodiment and mode of operation of thepresent invention have been described in the foregoing specification.However, the invention which is intended to be protected is not to beconstrued as limited to the particular embodiments disclosed. Further,the embodiments described herein are to be regarded as illustrativerather than restrictive. Variations and changes may be made by others,and equivalents employed, without departing from the spirit of thepresent invention. Accordingly, it is expressly intended that all suchvariations, changes and equivalents which fall within the spirit andscope of the present invention as defined in the claims, be embracedthereby.

What is claimed is:
 1. A crystal unit, comprising: an AT-cut crystalelement, having a planar shape which is approximately a rectangularshape, excitation electrodes, disposed on front and back of principalsurfaces of the AT-cut crystal element; and extraction electrodes,extended from the excitation electrodes to a side of one side of theAT-cut crystal element via a side surface of the AT-cut crystal element,wherein assuming that an extraction angle of the extraction electrodefrom the principal surface to the side surface is defined as an angle θwith respect to an X-axis of a crystallographic axis of a crystal of theAT-cut crystal element, the angle θ is equal to or greater than 59degrees and equal to or less than 87 degrees.
 2. The crystal unitaccording to claim 1, wherein angle θ is equal to or greater than 62degrees and equal to or less than 75 degrees.
 3. The crystal unitaccording to claim 1, wherein the angle θ is equal to or greater than 64degrees and equal to or less than 74 degrees.
 4. The crystal unitaccording to claim 1, wherein the side surface is an inclined portioninclined such that the AT-cut crystal element decrease in thickness toan end of the AT-cut crystal element along a Z′-axis of thecrystallographic axis of the crystal, the side surface having threesurfaces which are a first surface, a second surface and a thirdsurface, the first surface is an inclined portion of a surfacecorresponding to a surface where an X-Z′ surface as the principlesurface expressed by the crystallographic axis of the crystal of theAT-cut crystal element is rotated by 4°±3.5° with an X-axis of thecrystal as a rotation axis, and the extraction electrode is extractedvia the first surface.
 5. The crystal unit according to claim 4, whereinthe first surface, the second surface, and the third surface intersectin this order, the second surface is a surface corresponding to asurface where the principal surface is rotated by −57°±5° with theX-axis of the crystal as a rotation axis, and the third surface is asurface corresponding to a surface where the principal surface isrotated by −42°±5° with the X-axis of the crystal as a rotation axis.